Would a rail pressure gauge show this? If so someone on here must have both a fuel press. gauge and a rail press. gauge? I will soon.

I was thinking about disconnecting my in-tank lift pump to see if the CP3 could suck fuel through it. I may hook up a gauge that will measure vacuum on the fuel line to see how hard it has to pull to get fuel in that case, and compare to a Duramax with clean filters.

 

I have a fuel pressure gauge and a rail pressure gauge. I always have 10 psi of pressure (everyday driving) or can turn it up to 30+psi if deemed necessary. Whatever I set my pressure at, it remains from idle to WOT. Can't help you with the zero psi thing. Honestly, the rail gauge fluctuates quite a bit so it would be hard to monitor something like that unless you have a passenger keep an eye on it for you.

 

if the cp3 pump indeeds suck fuel...then i would hope that our LP would have a by pass safty feature on it......if not then i would put a Y in the fuel line with a high flow check valve so it can by pass if the LP quits....

 

Well rbox, I say give it a try and disconnect the intake pump. Let us know how it goes. You have got me curious, so please.... be our Guinea pig..... seriously.

 

Of course, the VP44 will suck fuel too, but the question is, will the CP3 be damaged or life shortened by sucking fuel?

 

You got a spare CP3 laying around to overnight me incase mine craps out?

 

No, but I see them all the time on Ebay for around $250. Gaurenteed to work. So they say. There feed back ratings are 100%. Give it a try.

 

It shouldn't... lubrication and cooling from incoming diesel fuel are priorities with the cp3 as opposed to the vp. If it doesn't have enough fuel for those two issues, it will just kill the engine. No harm done.

 

the way i look at it is that the lift pump has a performance curve like any other pump. Lets say you graph head or psi in this case on the vertical axis and flow volume in gallons/minute on the horizontal. The way pumps work is that they produce more head or pressure when they flow less volume/unit time, and as a pump flows more volume, the pressure they produce goes down. So when you are at idle, you see 10psi and the lift pump isn't flowing that much. When you step on it and the psi on your gauge goes down its because the pump is flowing more. At 0-1 psi the pump is flowing its maximum, and as long as that flow volume is above a specified amount dodge says its ok regardless of the psi. With 1 psi the pump is still able to produce enough head to overcome friction loss in the lines and account for the height above the pump in the tank to the input on cp3.

Now if your heavily modded and you suck your gauge down to zero and start creating a vaccume, then yes you need a pump that can produce more flow at a higher pressure so the cps doesn't have to suck fuel. But 1 - 0 psi doesn't mean that the lift pump isn't flowing enough fuel, it just means its on the outer limit of its performance curve!

 

Knowing your baseline numbers is also a good way to know what shape the pump's in. You'll know it's time for a new pump at some point as the numbers drop over time, hopefully before a total failure.

 

I am running 3 filters on my stock LP. A Baldwin 20 mic f/w sep, Baldwin PF7977 in the OE canister, and Cat 2 min between the OE canister and the CP3. I have a big-line kit and hi-flow banjos. At the CP3 I see 9psi at idle, 7.5 +- 1psi for 99% of driving. The lowest I have seen it was this weekend, 2400 rpms, 20K rail psi, 32 psi boost, 90% load and still had 5.5psi. This is all on stock fueling.

 

So, in reading about the FASS and Airdog I see that they supposedly remove air that is trapped in the fuel. Is air in fuel a problem with the Dodge's with a working lift pump, or is that more of an issue with the Duramax's that suck fuel?

Does the FASS or Airdog really make a difference on a stock truck? Does it alter the injector spray pattern as much as claimed?